1. Field of the Invention
This invention relates to the art of evaporation of the solvent or eluent of liquid fractions, such as those obtained from extraction or liquid chromatography, whose evaporation is necessary for analysis of the substances dissolved or carried therein. Specifically, this invention comprises an apparatus and a method for evaporating such solvents by application of a heated jet of non-reactive gas to the surface of the sample to be evaporated.
2. Description of the Prior Art
In the practice of analytical chemistry, the weighing and subsequent analysis of isolated fractions of substances undergoing testing or identification is frequently a critical step. This frequently requires evaporation of the solvent or eluent of the fractions. The usual method for evaporating simple liquid fractions is to place them in a beaker or other laboratory vessel and then place the vessel on a hot plate or the like to heat it from underneath. A frequently observed phenomenon when this type of heating is practiced is the tendency of the liquid fraction sample to creep up the walls of the container and out. To overcome this problem, a special beaker is sometimes employed. This beaker, an "inverted rim beaker", is expensive, fragile, difficult to weigh and difficult to clean. The use of such beakers requires subsequent redissolving of the fraction and complicated weighing procedures in order to secure reliable results.
Some fractions separate into two or more phases with the non-volatile oil phase on top. When heat is applied beneath these samples, the lower volatile phase may evaporate explosively, causing a loss of the sample to be evaporated and putting a halt to subsequent analysis. Types of mixtures which present this practical obstacle to analysis are often encountered in laboratory work.
Due to the problem described, such fractions are usually evaporated with a rotary evaporator. The rotary evaporator works on the theory that evaporation is best achieved with a large surface area and under a vacuum. The fraction is manually transferred to a large round-bottomed flask. A liquid bath is carefully jacked up under the flask and the motor of the evaporator is started. Vacuum is carefully applied to the flask and regulated manually by observing the evaporation. Too little vacuum results in the evaporation taking too long, and too much vacuum results in the fraction blowing over into the condenser and slop flask. Additionally, the rotary seal of the evaporator commonly requires numerous bearings and gives a very short useful life, on the order of six months. While the seal is wearing out, the grease and debris therefrom may end up in the fraction, spoiling subsequent analysis.
When evaporation with a rotary evaporator is completed, the residue is spread in a thin film over the large inside surface of the flask. The outside is coated with a film of dirty bath liquid. The motor is turned off and the bath is jacked down. The outside of the flask must then be cleaned. To remove the flask, heating with a torch may be required if the joint sticks. Frequently, the heating may result in cracking, and as a result, grease gets inside the flask, and admitted air may oxidize the residue. The residue is then redissolved in a small amount of solvent, transferred to a smaller weighable container and evaporated again while being watched to make sure that the residue does not overheat, after the solvent has been removed.
In prior art practice, therefore, the skilled chemist is frequently presented with fractions whose weighing and subsequent analysis is critical, but to whom a practical means of properly preparing the fraction is unavailable, making it difficult and expensive to secure reliable results.
I have therefore invented an apparatus and method for evaporating such liquid fractions which does not require the use of equipment that is expensive and difficult to handle, does not require manual observation and control, does not require difficult, tedious or repetitive manual steps to isolate the fraction, and, at the same time, provides uncontaminated residues that may be easily weighed and analyzed.